Point-defect energies in the nitrides of aluminum, gallium, and indium

Abstract

Experimental data on the nature and energetic location of levels associated with native point defects in the group-III metal nitrides are critically reviewed and compared with theoretical estimates. All three show strong evidence of the existence of a triplet of donorlike states associated with the nitrogen vacancy. Ground states are at about 150, 400, and 900 meV from the conduction-band edge in InN, GaN, and AlN, respectively, with their charged derivatives lying closer to the band edge. These values agree with both modified-hydrogenic and deep-level calculations, surprisingly well in view of the inherent approximations in each in this depth range. The InN donor ground state is both optically active and usually occupied, showing a distinctive absorption band which is very well described by quantum-defect analysis. Variation of threshold with electron concentration shows a Moss-Burstein shift commensurate with that observed in band-to-band absorption. In both GaN and AlN, levels have been identified at about 1/4${\mathit{E}}_{\mathit{G}}$ and about 3/4${\mathit{E}}_{\mathit{G}}$, which correlate well with predictions for the antisite defects ${\mathrm{N}}_{\mathit{M}}$ and ${\mathit{M}}_{\mathrm{N}}$, respectively, while similar behavior in InN is at odds with theory. The metal-vacancy defect appears to generate a level somewhat below midgap in AlN and close to the valence-band edge in GaN, but has not been located experimentally in InN, where it is predicted to lie very close to the valence-band edge. A tentative scheme for the participation of two of the native defects in GaN, namely ${\mathit{V}}_{\mathrm{N}}$ and ${\mathrm{N}}_{\mathrm{Ga}}$, in the four broad emission bands found in Zn-compensated and undoped GaN is offered.